Insular dwarfism

Skeleton of
Malta's extinct Palaeoloxodon falconeri, the smallest known species of
elephant. Adult males measured less than one meter in shoulder height and weighed about 305 kg. Females were considerably smaller.

Insular dwarfism, a form of
phyletic dwarfism,[1] is the process and condition of the reduction in size of large animals over a number of generations[a] when their population's range is limited to a small environment, primarily islands. This natural process is distinct from the intentional creation of dwarf breeds, called
dwarfing. This process has occurred many times throughout evolutionary history, with examples including
dinosaurs, like Europasaurus, and modern animals such as
elephants and their relatives. This process, and other "
island genetics" artifacts, can occur not only on traditional islands, but also in other situations where an ecosystem is isolated from external resources and breeding. This can include
caves, desert
oases, isolated valleys and isolated mountains ("
sky islands"). Insular dwarfism is one aspect of the more general
"island effect" or "Foster's rule", which posits that when mainland animals colonize islands, small species tend to evolve larger bodies, and large species tend to evolve smaller bodies.

Possible causes

There are several proposed explanations for the mechanism which produces such dwarfism.[3][4]

One is a selective process where only smaller animals trapped on the island survive, as food periodically declines to a borderline level. The smaller animals need fewer resources and smaller territories, and so are more likely to get past the break-point where population decline allows food sources to replenish enough for the survivors to flourish. Smaller size is also advantageous from a reproductive standpoint, as it entails shorter
gestation periods and
generation times.[3]

Among herbivores, large size confers advantages in coping with both competitors and predators, so a reduction or absence of either would facilitate dwarfing; competition appears to be the more important factor.[4]

Among carnivores, the main factor is thought to be the size and availability of prey resources, and competition is believed to be less important.[4] In
tiger snakes, insular dwarfism occurs on islands where available prey is restricted to smaller sizes than are normally taken by mainland snakes. Since prey size preference in snakes is generally proportional to body size, small snakes may be better adapted to take small prey.[5]